Woven heat exchanger

ABSTRACT

In a woven ceramic heat exchanger using the basic tube-in-shell design, each heat exchanger consisting of tube sheets and tube, is woven separately. Individual heat exchangers are assembled in cross-flow configuration. Each heat exchanger is woven from high temperature ceramic fiber, the warp is continuous from tube to tube sheet providing a smooth transition and unitized construction.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. DE-AC07-76ID01570 between the U.S. Department of Energy andEG&G Idaho, Inc.

BACKGROUND OF THE INVENTION

This invention relates generally to a heat exchanger for waste heatrecovery from high temperature industrial exhaust streams.

The Environmental Protection Agency (EPA) estimated that 23% of themajor or energy intensive industrial fuels and electricity energyconsumption is discharged as waste heat in flue gases. Industry hasattempted to decrease the amount of this lost energy with waste heatrecovery systems (recuperators or regenerators). However, because of thehigh temperatures and corrosive constituents in the waste heat streams,lack of durability of the construction materials has limited recovery.Recovery of waste heat from high-temperature industrial gas streams ismost commonly done by preheating combustion air using either arecuperator or a regenerator. Generally, a a recuperator uses theexhaust gas to heat the combustion air directly or through a partitionwall. A regenerator normally allows hot exhaust gas and combustion airto move alternately through the same passage, thus indirectly heatingthe combustion air. Historically, recuperators and waste heat boilershave been constructed of metal. However, these conventional metallicheat exchangers cannot survive for extended periods in high-temperature,dirty environments without incurring severe performance penalities.Failure of the system affects not only its ability to recover wasteheat, but also the operation of the process to which it is attached.Corrosion has debilitating effects on most all metals, causing prematurefailures and/or excessive leakages. Fouling decreases heat transferrate, increases pressure drop, and adds expense due to increased surfacearea and the necessity for cleaning and refurbishing.

Ceramics, an alternative to metals, allow significantly higher materialtemperatures and offer resistance to many corrosive constituents inindustrial waste streams. However, certain technical limitations existwhich severely restrict the application of advanced ceramic heatexchangers in high temperature fouling and corrosive waste streams.These include the high costs for ceramic fabrication, problems withsatisfactorily joining ceramics, the lack of data and accurate methodsto predict thermal-structural behavior of ceramics especially asaffected by long-term exposure to corrosive environments, thesensitivity of some of the more corrosive-resistant ceramics to thermalshock fracture, and the inability to detect and evaluate flaws causingfailure.

Some major waste heat streams include glass melting, aluminum remelt,and steel soaking and reheat furnace flue gases. Although many othersources of waste heat exist, these three represent the largest quantityof waste heat. The streams are very high temperature, generally 1500° to2800° F. and usually contain highly corrosive constituents which degradethe materials and contain particulates that tend to build up and foulthe heat exchanger.

Therefore it is an object of the present invention to provide a ceramicheat exchanger suitable for use in waste heat streams.

It is another object of the present invention to provide a heatexchanger suitable for use at high temperatures and resistant tocorrosion and fouling.

Additional objects, advantages, and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects and in accordance with thepurposes of the present invention, a tube-in-shell heat exchanger maycomprise a first tube sheet; a second tube sheet; and a tube extendingfrom the first tube sheet to the second tube sheet, the tube sheets andtube being formed of woven ceramic fibers. Preferably, the ceramic warpfibers extend from each tube sheet to the tube forming a continuoustransition from the first tube sheet to the tube and from the secondtube sheet to the tube. The tube sheet being woven with the same fibersthat form the tube warp allows a smooth load carrying transition fromtube to tube sheet with a resulting low pressure drop. This also permitsthe tube sheet to have the same thickness as the tube itself, minimizingany thermally induced stresses from discontinuities in the transitionarea. The thin tube sheet will also be an effective heat transfersurface.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated in the accompanying drawingswherein:

FIG. 1 is an isometric of three woven tube-in-shell heat exchangers,each having a single tube.

FIG. 2 is a top view of the weaving pattern on the tube sheet for ninetube-in-shell heat exchangers assembled together.

FIG. 3 is a cross section of an operative heat exchanger.

FIG. 4 is a view showing one means of weaving a heat exchanger tube.

FIG. 5 is a view showing another means of weaving a heat exchanger tube.

DETAILED DESCRIPTION OF THE INVENTION

The woven tube ceramic heat exchanger is a basic tube-in-shell design,differing from the traditional design in the materials fabricationmethods used, and the configuration of tubes and tube sheets. Referringto FIG. 1 three tube-in-shell heat exchangers 10 are shown. In thisembodiment, each heat exchanger is woven separately and then theindividual heat exchangers are assembled in a cross-flow configuration.Each heat exchanger is woven from high temperature ceramic fiber into aone piece tension structure composed of a tube and tube sheets. The warpis continuous from tube to tube sheet providing a smooth transition. Theweft is partly shown for clarity and extends continuously from tube totube sheet surface. Alternatively, more than one tube could be woveninto the tube sheets. In FIG. 2 there are shown nine heat exchangers 10from a top view as they would be assembled together to form a largerheat exchanger.

Referring to FIG. 3, there is shown one embodiment of a completeoperative cross flow heat exchanger incorporating the particular tubeswhich are the subject this disclosure. The components are containedwithin a recupurator shell 12 within which is provided a centralpassageway 14 into which exhaust gas enters and out of which exhaustexits after passing between individual tubes 16. The tubes are formed asotherwise herein disclosed. Combustion air enters the assembly at inletport 18 and passes through half the pipes via inlet plenum 20.Combustion air then flows through the other half of the tubes 16 viacombustion air plenum 22 as indicated by the arrows and leaves theexchanger via outlet plenum 24 and output port 26. Each tube 16 isconnected in a matrix to top tube sheet 28 and bottom tube sheet 30 asshown for example in FIG. 2.

Weaving is already an automated, well-established industry and onlyrelatively minor changes, if any, are needed to handle ceramic fiber,mainly related to fiber flexibility. The computer andcomputer-controlled looms make possible a great variety of designvariations. Tubes can be made in many shapes or configurationsdetermined to be effective for a specific waste stream. Tubes can bearranged as desired for fouling and corrosion resistance, ease ofcleaning and inspection, and stress field optimization or redirection.Heat exchanger design for each application can be easily optimized withthe aid of the computer controlled loom if a detailed analysis model isdeveloped.

An alternate construction method may be used in cases where heatexchangers are "built up" from three parts: a flat woven tube sheet withholes woven or cut, a woven/braided tube, and a transition piece toconnect the two. Rather than weave the heat exchanger as a unit, modulesof the heat exchanger may be woven and then joined together.

The one-piece (unitized) construction has the potential to eliminateleaks between tubes and tube sheets, and the uniform material thickness(preferably 0.060 in thick or less) has the potential to eliminatethermal gradient stress caused by material thickness changes. Theradiused transition from tube to tube sheet as seen in FIG. 1 permitsthe smooth transfer of loads, from pressure or thermal expansion,without stress concentration such that stress fields are more constantand change more gradually when they do change. If some fibers have flawsor are overstressed and break, others carry the load and the recuperatordoes not completely fail, as would a unit made of monolithic ceramicmaterial. Another advantage of the woven heat exchanger is that the thinand relatively lightweight fabric uses only the required amount ofmaterial which helps to minimize costs.

Some commercially available ceramic fibers include silicon carbide(available as Nicalon® SiC continuous fiber--500 filament), alumina(available as Fiber FP® yarn--200 filament/yarn) and zirconium oxide(stabilized). (Nicalon is a Nippon Carbon Co., Trademark and FP is aDuPont Trademark.)

Since ceramics display excellent compressive strength and fibers haveexcellent tensile strength, the woven tube-in-shell heat exchanger canbe built to withstand greater compressive stresses by embedding the heatexchanger in a matrix, as shown in FIG. 4, or the individual strands canbe coated with a ceramic material after weaving. In FIG. 4, the warpstrands 34 are woven with the weft strands 36 and are contained withinmatrix 38.

If a ceramic fiber/matrix is susceptible to corrosion by a particulargas stream constituent, the matrix can be coated with a resistantmaterial or another compound can be added to to the matrix. In this way,a matrix or coating can be compounded to exhibit specific properties, orstratified for special chemical resistances. Some suitable matrixmaterials include zirconia and stabilized zirconia types.

The woven heat exchanger lends itself to the techniques of NDE andacoustic emission, especially since the diameter of all ceramic fibersin a heat exchanger need not be the same. If tracer strands, sized tobreak well before the main fibers, are included in the weave, acoustictransducers can be used to detect for fractured fibers and identify setpoint stress levels and their location. This information can be used topredict maximum safe stress versus operating condition variables, stressfield concentrations, and to indicate impending failure. This techniquecan also be used to indicate when fatigue limits are imminent andreplacement is required.

Varying fiber diameters instead of fiber materials is also useful forheat transfer. The ratio of surface roughness to hydraulic diameter fora specific operating condition can be varied to yield the desired heattransfer coefficient and pressure drop. Braiding or weaving a largediameter fiber spirally into a tube will act as a turbulator. Adding ormaking fins integral to the tube structure is possible, but addscomplexity and cost. Weaving wooly yarn strands to act as pin fins ismuch simpler. As shown in FIG. 5 the weft strand 40 can be coiled abovewooly fibers 42 with the warp strands 44 interspersed with weft strand40. On the gas side of the heat exchanger, the wooly fibers induceturbulence in addition to acting as pin fins. Properly positioned woolyfibers delay boundary layer separation and may damp vortex sheddingoscillations.

In long tubes, weft threads take the hoop stress and could have asmaller cross section than warp threads, which run longitudinally. Warpthreads bear tube weight and transfer loads to the tube sheet. At thetransition area between tubes and tube sheet, weft threads could beincreased in size to stiffen the transition area from tube to tubesheet. With the unitized tube design, computer-controlled looms can makealmost any smooth transition geometry.

Chemical attack by gas stream constituents can be minimized by coatingthe fibers with an impervious material such as high viscosity glasses,alumina, and mullite. Embedding the ceramic fibers in a rigid matrixdecreases buckling of the unsupported fibers and also preventsenvironmental attack.

No matter how tightly a fiber is woven, some gaps between fibers willoccur resulting in leakage. However, leakage can be effectivelyeliminated by the choice of material for the ceramic fibers.

For example, SiC forms an oxide coating on its outer layer at hightemperatures, and zirconia fibers sinter to each other above 2500° F.These coating/binding mechanisms seal the pores between fiberseliminating leakage. Leakage can also be eliminated with a viscous glassin the gaps, or a matrix encapsulating the fibers. A viscous glasscoating has the additional benefit of dampening oscillations, and arigid matrix also stiffens the structure.

The woven tube-in-shell design has several inherent advantages to limitfouling. These advantages include the relatively large passages that donot readily plug and flexibility of design which allows for smoothcontours to minimize dead regions where fouling occurs.

EXAMPLE DESIGN CALCULATION FOR A WOVEN HEAT EXCHANGER

The following analysis was completed to size a ceramic woven heatexchanger to be used as a recuperator in a typical steel soaking pit.The stream conditions and performance parameters for this applicationare given in Table 1. Table 2 summarizes the calculated geometry andperformance. A summary of the analysis method is given below.

Analysis of the woven tube designs is predicated on the followingassumptions: the tube to tubesheet transition is well-rounded resultingin entrance and exit flow coefficients of 0.04 and 1.0 respectively(Crane: Flow of Fluids p. A-29); there is heat transfer across thetubesheets; rectangular ducts are used with round tubes axially orientedalong the height dimension (i.e., running top to bottom not side toside) the loss coefficient due to turning (for multiple pass units) is≈3.6 for each turn (Idel'Chik: Handbook of Hydraulic Resistance, diag.6-21) based on rectangular turning ducts of 1.0 aspect ratio and abetween pass length of zero; and a staggered tube arrangement.

Analysis was performed on a HP-41C calculator using a programspecifically developed using the above assumptions. Initially, geometryparameters, air and gas stream conditions, and air/gas/materialproperties, and the maximum allowed gas ΔP are input. After calculatingthe number of tubes in the first row and other assorted flow variables,the maximum number of total tube rows is calculated based on gas ΔP max(Kreith: Principles of Heat Transfer, eq. 9-11 and 9-12). The user thenchooses the number of air side passes and the total number of tube rowswhich does not exceed the maximum number. The only restriction is thatan integer value will result when dividing the total number of rows bythe number of passes. The gas side ΔP is then calculated based on thetotal number of rows. If the gas side Reynolds number is below 1000, theuser is referred to FIG. 9.18 (Kreith: Principles of Heat Transfer) forthe gas side friction factor. The air side ΔP is then calculated bymanipulation of Darcy's formula for heat loss. Air side friction factorsare calculated by one of four single phase correlations from the TRACcomputer code. The applicable correlation is chosen based on Reynoldsnumber and relative roughness, ε/D. Absolute roughness, ε, can be inputby the user or for ceramic coated woven fabric defaults to 0.001 inch.fL/D or K_(pipe) is added to the other loss coefficients --K_(ent),K_(exit), and K_(turn) (as specified by the geometry) and and thenK_(total) is input to the manipulated Darcy formula to get air sidepressure drop. At this point, the user can modify the geometry and startthe analysis over if the pressure drops are not satisfactory or continueif they are.

Continuing, the gas and air heat exchange areas are calculated. If thegas Reynolds number is less than 1000 the user must look up theColburn-j factor from FIG. 9-18 (Kreith: Principles of Heat Transfer),whereupon the gas film coefficient is backed out. For Reynolds numbergreater than 1000, equation 9-10 (Kreith: Principles of Heat Transfer)is used to get the gas film coefficient. Next, the tube length iscalculated assuming a shape factor of 1.0 and gas emissivity andabsorptivity must be input. Since radiation varies as the 4th power ofthe temperature, a 4th power average is used for average gas temperaturein the standard radiation equations 5-66 and 5-67 (Kreith: Principles ofHeat Transfer). Unit gas thermal resistance is then the inverse sum ofthe gas film, fouling factor, and radiation coefficients. Unit thermalconductive resistance is the material thermal conductivity divided bythe conduction path length. For air side, Reynolds numbers less than2100, equation 8-28 (Kreith: Principles of Heat Transfer) is used tofind the air film coefficients. For Reynolds numbers greater than 2100,equation 8-21 (Kreith: Principles of Heat Transfer) is used. The airside unit thermal resistance based on gas side area is the inverse ofthe air side film coefficient times the air side/gas side Hx area ratio.The three resistances are added together. The inverse of this sum is theoverall unit conductance based on gas side Hx area, U. When multipliedby the gas side Hx area, the output is UA_(calc). If the answer issatisfactory, the material volume of the Hx can be calculated.Otherwise, some or all of the geometry variables can be changed and thecalculation rerun.

In this example the materials volume is significantly less than for aconventional ceramics design, resulting in significant cost savings formaterials.

                                      TABLE 1                                     __________________________________________________________________________    Stream Conditions and Performance Parameters                                  for a Typical Steel Soaking Pit (with 2% Excess Air)                          STREAM CONDITIONS & PROPERTIES                                                                           DESIGN  MAXIMUM                                    __________________________________________________________________________    AIR .m            LBM/HR   8654    13422                                      GAS .m            LBM/HR   9114    14130                                      AIR T in          °F.                                                                              100     100                                       GAS T in          °F.                                                                             2450    1700                                       AIR T out         °F.                                                                             2000    1420                                       GAS T out         °F.                                                                              906     612                                       AIR ρ bulk    LBM/FT.sup.3                                                                           .0264   .0325                                      GAS ρ bulk    LBM/FT.sup.3                                                                           .0187   .0249                                      AIR μ bulk     LBM/FT-SEC                                                                             2.5230 × 10.sup.-5                                                              2.22060 × 10.sup.-5                  GAS μ bulk     LBM/FT-SEC                                                                             3.1600 × 10.sup.-5                                                              2.6353 × 10.sup.-5                   AIR k bulk        BTU/HR-FT-°F.                                                                   .0327   .0279                                      GAS k bulk        BTU/HR-FT-°F.                                                                   .0425   .0344                                      MATL k            BTU/HR-FT-°F.                                                                    10      10                                        AIR Cp bulk       BTU/LBM-°F.                                                                     .2694   .2600                                      GAS Cp bulk       BTU/LBM-°F.                                                                     .3147   .2996                                      PERFORMANCE PARAMETERS     .809    .825                                       EFFECTIVENESS,                                                                # OF AIR SIDE PASSES ε                                                                           3                                                  # OF TRANSFER UNITS, N.sub.ω                                                                       ˜5.2                                                                            ˜5.7                                 MINIMUM UA required                                                                             BTU/HR-°F.                                                                      12123   19891                                      MAXIMUM GAS ΔP                                                                            INCHES W.C.                                                                             8.0                                               MAXIMUM AIR ΔP                                                                            INCHES W.C.                                                                            30.0                                               __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Geometry and Performance for Ceramic (Sic)                                    Heat Exchanger in a Typical Steel Soaking Pit                                 DESIGN REQUIREMENTS       DESIGN                                                                              MAXIMUM                                       __________________________________________________________________________    MAXIMUM GAS ΔP  INCHES W.C.                                                                        8.0                                                MAXIMUM AIR ΔP  INCHES W.C.                                                                       30.0                                                MINIMUM UA required BTU/HR-°F.                                                                   12123 19891                                         FOULING FACTOR             0.0                                                WOVEN TUBE HX DESIGN: GEOMETRY & PERFORMANCE                                  DUCT HEIGHT × WIDTH                                                                      INCH × INCH                                                                      48 × 22                                       TUBESHEET LENGTH/PASS                                                                          INCH     65.88                                               TUBE O.D. × WALL                                                                         INCH × INCH                                                                      .450 × .080                                   STAGGERED TUBE PITCH                                                                           INCH      1.10                                               # OF TUBES/PASS           1380                                                GAS SIDE Hx AREA FT.sup.2 2008.27                                             MATERIAL VOLUME  FT.sup.3  6.0105                                             GAS ΔP calc                                                                              INCHES W.C.                                                                            4.38  7.18                                          AIR ΔP calc                                                                              INCHES W.C.                                                                            17.18 30.07                                         U calc (on gas side area)                                                                      BTU/HR-FT.sup.2 -°F.                                                            7.83  9.93                                          UA calc          BTU/HR-°F.                                                                      15733 19929                                         __________________________________________________________________________

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A tube-in-shell heatexchanger comprising:a first tube sheet, a second tube sheet, and a tubeextending from the first tube sheet to the second tube sheet, the tubesheets and the tube being formed of woven ceramic fibers, said woventube having longitudinally oriented warp fibers extending from each tubesheet to the tube forming a continuous transition from the first tubesheet to the tube and from the second tube sheet to the tube.
 2. Theheat exchanger of claim 1 wherein the ceramic fibers are coated with aceramic matrix material after weaving.
 3. The heat exchanger of claim 2wherein the matrix is formed of a viscous glass.
 4. The heat exchangerof claim 1 wherein the ceramic fibers are formed of a material selectedfrom the group consisting of silicon carbide, alumina, and zirconiumoxide.
 5. The heat exchanger of claim 1 wherein the thickness of thetube wall and the tube sheets is 0.060 in or less.
 6. The heat exchangerof claim 1 further comprising metal warp fibers interspersed among andfused to the ceramic warp fibers.
 7. The heat exchanger of claim 1further comprising one or more wooly fibers woven into the tube toinduce turbulence.
 8. The heat exchanger of claim 1 comprising aplurality of hollow tubes extending from the first tube sheet to thesecond tube sheet.
 9. The heat exchanger of claim 1 wherein a largediameter fiber is spirally woven into the tube wall to act as aturbulator.
 10. The heat exchanger of claim 1 further comprising atransition piece for connecting each tube sheet to the tube, the tube,tube sheets, and transition pieces being individually woven.
 11. A heatexchanger comprising a plurality of tube-in-shell heat exchangersassembled in cross-flow configuration, each tube-in-shell heat exchangercomprising:a first tube sheet; a second tube sheet; a tube extendingfrom the first tube sheet to the second tube sheet the tube sheets andthe tube being formed of woven ceramic fibers, said woven tube havinglongitudinally oriented warp fibers extending from each tube sheet tothe tube forming a continuous transition from the first tube sheet tothe tube and from the second tube sheet to the tube.
 12. The heatexchanger of claim 11 wherein the ceramic fibers are formed of amaterial selected from the group consisting of silicon carbide, alumina,and zirconium oxide.
 13. The heat exchanger of claim 12 wherein theceramic fibers are coated with a ceramic matrix material after weaving.